State Department travel advisory issued 15 June reads, in part:

"Reconsider travel to Russia due to terrorism and harassment. Some areas have increased risk.

Do not travel to:

- The north Caucasus, including Chechnya and Mount Elbrus, due to civil unrest and terrorism.
- Crimea due to foreign occupation and abuses by occupying authorities.

Terrorist groups continue plotting possible attacks in Russia. Terrorists may attack with little or no warning, targeting tourist locations, transportation hubs, markets/shopping malls, and local government facilities. Bomb threats against public venues are common."

According to cybersecurity firm Sixgill, ISIS has threatened Russians: "We shall slay you in your homeland" and “It will be a massacre that has never been seen in history before.”

The Russian government says it's taking "extraordinary measures" to combat any terrorist threat to the World Cup matches.

There's more World Cup-related information from the State Department here:

https://travel.state.gov/content/travel/en/international-travel/before-you-go/worldcup.htm